Definitively answering these questions about the difference between 9mm and .40 S&W would require very accurate real-time info about exactly how the rounds act from point of entry and exactly how they affect the body and mind of the attacker for the duration of the event.
Ultimately, this all boils down to: "No one has been able to demonstrate a practically significant benefit to using the .40S&W in real-world shootings."
The key is that it hasn't been for lack of trying. People have been trying to demonstrate the benefits of one caliber over other calibers for decades. They've looked at thousands of shootings and still the answer is the same. There are other variables that affect the outcome so much more strongly that there's no way to differentiate between the common service pistol calibers when trying to determine how terminal effect changes the outcome of real-world shootings.
This is why we tend to infer from other media.
Which is all well and good until we realize that even after trying very hard to do so for quite a long time, no one has been able show a correlation between the results inferred from other media testing of common service pistol calibers and the practical effects due to terminal ballistics on real-world shootings with those same calibers.
Again, the question comes down to this: How much weight do we put on an effect that NO ONE has been able to demonstrate in spite of decades of trying when we know that there are easily demonstrable benefits to switching?
The actual question is: does more power in a handgun bullet enhance ones ability to do meaningful damage in a gun fight and give one an edge in stopping a fight. The answer is yes.
You
believe (and other people
believe) that the answer is yes, but if there really were a definitive source who could demonstrate with hard evidence that the answer really was yes, we wouldn't be having this discussion and LE wouldn't be trending away from the .40S&W.
The fact is that no one has been able to prove the answer is really yes. They can "infer" that it might be yes from testing with other media, but when it comes down to actually demonstrating that it provides an edge in real-world shootings, the evidence just isn't there.
Why would the military do this? It's to enhance the potential effectiveness of the bullets with more power.
Since the article says the goal was to enhance the ability of the ammunition to penetrate deeply at longer distances, I'm going to go with that answer.
It's also worthwhile to keep in mind that the military is using ammunition that is very different from common LE issue/self-defense ammunition, and is likely far more concerned about performance against body armor.
The 40 S&W is a more powerful round than the 9mm. There is no argument about that. If the shooter can shoot the gun well, then more power is a useful option?
If it costs you NOTHING, then yes, why not go with more power. If something is free, there's no need to prove that it's worth the cost.
But if it costs something, then the logical/rational approach is to see if what it costs is worth what you get in return. It's very easy to show what it costs. But even after thousands of shootings and considerable effort expended, no one has been able to show what it buys in terms of real-world shootings.
It makes no sense to pay for a benefit that can't be shown to actually exist. That's why the FBI gave up on the .40S&W and why LE is moving away from it in general.
It is apparent to me that this general idea steps on some folks' toes, but it's purely based on common sense.
If I told you to pay me $100 in return for a benefit, you would reasonably expect me to provide proof that the benefit existed. If I were unable to demonstrate the benefit, why would you pay me the $100? That's the situation we have here. The cost is clear, the benefit isn't. People can (and have) hand-waved the potential benefits, or inferred benefits, but when it comes down to rubber-meets-the-road proof, there just isn't any.